The present invention relates to apparatus for applying an electrostatic charge to a moving web, in general, and to apparatus for establishing a predetermined uniform charge level on a moving web of plastic or plastic coated material, in particular.
In the manufacture or handling of web materials, positive and/or negative electrostatic charges appear on such materials for any number of reasons. In web coating operations, for example, the presence of non-uniform electrostatic charges may cause a nonuniform-thickness coating to be deposited on a moving web which may result in the production of a coated web which is unsuitable or unacceptable for its intended purpose. If electrostatic charges appear on a web that has already been coated with a photographic emulsion, a spark produced by the discharging of such charges may cause localized exposure of the emulsion.
In product assembly operations such as the assembly of photographic film units into film packs or cassettes of the type sold by Polaroid Corporation under its registered trademark SX-70 Land Film, it is essential that the charge level on each film unit placed in such a film pack be at a very low level (preferably zero) in order to avoid having adjacent film units electrostatically attracted to one another. Film units of this type must be moved out of their cassettes and through a pair of adjacent fluid-spreading rollers at an optimum rate of speed in order to insure proper film unit developing. If sufficient attractive forces develop between adjacent film units, it may become difficult or even impossible for the film unit drive system to overcome this attractive force and move the film unit at the approprate rate of speed for proper film unit developing.
Previously available web charge-controlling apparatus such as that disclosed in U.S. Pat. No. 3,730,753 to KERR include a plurality of spaced-apart corona sources that are energized from either AC or DC power sources. While this apparatus produces a relatively uniform electrostatic charge on a moving web, it requires that the output voltage of each power supply be at a level that will produce corona and that a plurality of spaced-apart corona producing electrodes be provided together with space and means for mounting same.
A primary object of the present invention is to provide apparatus for establishing a predetermined uniform charge level on a moving web of plastic or plastic coated material.
Another object of the present invention is to provide apparatus for establishing a predetermined charge level on a randomly charged moving web that includes both AC and DC power sources wherein only the output of one of said power sources must have a magnitude that will produce corona.
A further object of the present invention is to provide a single corona-generating electrode that will both neutralize random electrostatic charges on a moving web and establish a particular residual charge level thereon.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiment thereof, taken in conjunction with the accompanying drawings.